An oscillator circuit generates an output signal at a desired frequency with a specific waveform. Although an oscillator circuit can be used to perform definite functions in different type circuits, such as timing circuits, transmitter/receiver circuits and the like, for example, the desired frequency of the output signal is not typically changed.
There are numerous approaches for an oscillator circuit which uses semiconductor devices, one of which is the Pierce oscillator. The Pierce oscillator includes a crystal, an inverter connected across the crystal, and a respective capacitor connected to a first and second terminal of the external crystal. The inverter provides phase shift and amplification for the oscillating signal. An amplifier is connected either before or after the inverter for providing the output signal which may be used for performing a definite function, such as synchronizing digital logic circuitry. The amplifier has a large gain to accurately detect small changes in the crystal output voltage.
Unfortunately, manufacturing process variations in a metal oxide semiconductor (MOS) integrated oscillator circuit causes frequency dispersion in the output signal. For example, transistor and capacitor process variations cause the desired frequency of the output signal from the integrated oscillator circuit to vary. In addition, frequency dispersion of the integrated oscillator circuit can also be caused by temperature and voltage variations, and aging of the components. Frequency dispersion can be minimized to a certain point in the design of the integrated oscillator circuit. However, some applications require a very small dispersion, such as within 25 part per million (ppm). Consequently, the frequency of the output signal needs to be set or trimmed to the desired frequency.
One approach to setting the frequency of the output signal is to include capacitors having variable capacitance within the integrated oscillator circuit. By including two or more variable capacitors, they can be selectively connected for setting the frequency of the output signal to the desired frequency. For example, U.S. Pat. No. 4,827,226 to Connell discloses a tuning network for an integrated oscillator circuit that includes a variable capacitor, i.e., a varactor, as a shunt element for providing at least one type of adjustment of the oscillating signal. More than one type of adjustment can be provided by including a bank of varactors for each of the shunt elements of the tuning network, in which various individual varactors are selected in binary fashion for setting the output frequency.
Another approach is disclosed in U.S. Pat. No. 5,030,926 to Walden, which is assigned to the current assignee of the present invention. The frequency range over which the output signal of the integrated oscillator circuit is typically adjusted is increased by the addition of a variable capacitor directly to the amplifier within the oscillator circuit. The frequency range can be further increased by selectively connecting an additional pair of variable capacitors to the oscillator circuit.
Unfortunately, selectively connecting variable capacitances within an integrated oscillator circuit has several drawbacks. One drawback is that the control circuit selecting the variable capacitors is relatively complex. Consequently, an increased surface area of the integrated oscillator circuit is necessary to support a large number of components for implementing the complex control circuit. Another drawback is an undesired charge injection condition that results during switching between capacitors if the setting is performed while the integrated oscillator circuit is operating in an electronic system.